1. Field of the Invention
The present invention relates to a sheet stacking mechanism for successively stacking sheets being carried in from a carry-in opening on the upper surface of a sheet placing table, a sheet folding device, a sheet post-processing apparatus and an image forming apparatus provided with the sheet stacking mechanism.
2. Description of the Related Art
Some of image forming apparatuses include post-processing apparatuses such as a sheet middle-folding device as disclosed in Japanese Unexamined Patent Publication No. 2002-167120. A sheet stacking mechanism is provided in this sheet middle-folding device. However, since this sheet stacking mechanism has a mere construction of providing a pair of carry-in rollers at a carry-in opening for sheets, there is a likelihood that, upon receiving a sheet fed from the sheet post-processing apparatus, the trailing end of this sheet comes into contact with the leading end of a succeeding sheet, thereby causing a jam.
As means for solving such a problem, a processing tray includes a sheet stocking portion having a paddle for pressing a sheet in a guiding path for guiding the sheet, and a succeeding sheet is further stocked with the trailing end of the previously stocked sheet pressed by the paddle, for example, in Japanese Unexamined Patent Publication No. 2001-171889 although this technology is not applied to a sheet middle-folding device as described above. Accordingly, an occurrence of a jam caused by the collision of sheets can be prevented and even folded sheets can be stocked without causing any jam. This publication discloses that a high-speed processing can be carried out since the above makes it unnecessary to stop the conveyance of sheets during the application of stapling as a post-processing to a stack of sheets.
Further, Japanese Patent Publication No. 3423462 discloses the arrangement of a paddle for pressing an upstream end of a stapled stack of sheets with respect to a sheet discharging direction in a discharge tray.
However, even with the technologies disclosed in the latter two publications, if a sheet is stacked in an unstable state such as a curled state upon being stored, it is difficult to solve problems such as the switch of sheets (disorder in numbering) caused by a succeeding sheet having slipped in a clearance between the curled sheet and the sheet right below it or a tray and an occurrence of a jam caused by the collision of sheets. There have been also problems such as a loud hitting sound (noise) of the paddle when the stacked sheets are moved toward one end by the paddle in order to prevent the above switch of sheets.